It's clear that it failed. It's less clear that the reason is necessarily any of the ones you gave. Every dominant social network has been a walled garden, and, contrariwise, every attempt to create a major social network based around federation and open standards has, so far, failed.
Don't get me wrong, I'd love to see one succeed, and who knows, maybe Mastodon will overtake Facebook one day, but I don't see any evidence in the Google+ shutdown that suggests that this is the case.
